Cleanup: Mark some overridden methods with 'SK_OVERRIDE'.

src/effects/SkColorMatrixFilter.cpp

Index: src/effects/SkColorMatrixFilter.cpp
diff --git a/src/effects/SkColorMatrixFilter.cpp b/src/effects/SkColorMatrixFilter.cpp
index 32f94d924096536e346d95d4a771533e61db255a..2fa9e5d4ed21473007bc952c52421b4597aed3f2 100644
--- a/src/effects/SkColorMatrixFilter.cpp
+++ b/src/effects/SkColorMatrixFilter.cpp
@@ -416,7 +416,7 @@ private:
         this->initClassID<ColorMatrixEffect>();
     }
 
-    virtual bool onIsEqual(const GrFragmentProcessor& s) const {
+    virtual bool onIsEqual(const GrFragmentProcessor& s) const SK_OVERRIDE {
         const ColorMatrixEffect& cme = s.cast<ColorMatrixEffect>();
         return cme.fMatrix == fMatrix;
     }